In recent years, illumination systems that include multiple light sources are being developed. An example of such an illumination system is an arrangement of several sets of light sources in a structure (for example, a room, a lobby or a vehicle) that allows consumers to obtain a desired ambiance for the structure. The light sources, such as e.g. light emitting diodes (LED's), are driven electrically to produce light of a particular spectrum. Spectra of the individual light sources may differ from one light source to another, change over time, and depend on the drive level. Thus, in order to realize proper control of the illumination system, an accurate measurement of the light spectrum is necessary.
One approach to measuring the light spectrum of an illumination system is to use a spectrometer that includes an array of narrow-band color filters coupled to photodetectors. Every photodetector measures a small part of the spectrum filtered via the corresponding color filter. With the individual results from multiple photodetectors the entire spectrum can be reconstructed.
One type of a narrow-band color filter is an interference filter that includes two dielectric mirrors separated by a spacer layer. Although such a filter provides relatively high transmission at the desired wavelength and a very narrow response, this type of filter possesses an inherent drawback in that there are other wavelengths beyond the rejection band of the filter which are transmitted through the filter in addition to the desired wavelength (i.e., sidebands). In order to get rid of the sidebands, the filter must be combined with high pass and low pass filters. This adds to the complexity and cost of the devices including such filters. In addition, sidebands result in a situation where, in order to select the light component of the incident light to be transmitted, not only the thickness of the spacer layer needs to be varied, but also the thickness of the dielectric mirrors. This is problematic for spectrometer applications. For these applications it is desired to obtain as many different filter responses as are necessary in the visible part of the spectrum with as little variation in the layers as possible.
Another type of a narrow-band color filter is an interference filter that includes two metal mirrors separated by a spacer layer. Such a filter typically does not suffer from the sideband problems mentioned for the dielectric mirrors. Moreover, increasing the thickness of the metal mirrors allows narrowing the response. The transmission for such a filter is not nearly as high as one with dielectric mirrors, however because increasing the thickness of the metal mirrors also results in decreased transmission at the desired wavelength. In addition, silver, which is the most optimal metal from an optical point of view, has poor stability in ambient conditions. Therefore, additional packaging is typically required to protect silver, which, again, adds to the complexity and cost of the devices including such mirrors.
As the foregoing illustrates, there exists a need in the art for providing, an interference filter having high transmission at the desired wavelength, narrow response, large rejection band, and good stability in ambient conditions.